Semiconductor fingerprint sensing apparatus with shielding unit

ABSTRACT

Disclosed is a semiconductor fingerprint sensing apparatus with a shielding unit for sensing an inherent pattern of a fingerprint by using minute difference of fingerprint impedance. This fingerprint sensing apparatus has the shielding unit between a charge supplying unit and a sensing electrode to prevent DC current from being directly applied to the human body through the sensing electrode. The fingerprint sensing apparatus also has a discharging unit for compulsorily discharging charges residual in the shielding unit and the sensing electrode, so the sensing electrode may be always charged at a regular level.

TECHNICAL FIELD

The present invention relates to a semiconductor fingerprint sensor fordetecting an inherent pattern of a fingerprint by use of minutedifference of fingerprint impedances according to the shape of valleysand ridges of the fingerprint. In particular, the present inventionrelates to a fingerprint signal generating circuit applied to thesemiconductor fingerprint sensor.

BACKGROUND ART

The fingerprint sensing and matching technique is reliable for personalidentification or verification, and used in various fields. Afingerprint sensing method currently used in the fingerprintidentification system may be classified into an optical way and asemiconductor way, in broad.

The optical fingerprint sensor uses an image processing system whichscans rays to a fingerprint, extracts an inherent characteristic of thefingerprint and then compares it with well-known reference fingerprintcharacteristics. This image processing system generally has an opticalsensor for converting the fingerprint information into digitalwaveforms. In addition, the image processing system requires an opticaldevice, for example a laser source, a condenser and so on.

Conventionally, there have been published many patents disclosing suchan optical fingerprint sensor. For example, U.S. Pat. No. 4,210,899discloses an optic scanning fingerprint reader associated with a centralprocessing station for the purpose of the usage in security accessapplications which, for example, allow the approach of a person to acertain location or allow the access to a computer terminal.

U.S. Pat. No. 4,525,859 also discloses a video camera for determiningwhether a fingerprint is matched with an existing fingerprint databaseby use of details of the fingerprint, i.e., branches and terminations ofthe fingerprint ridges.

In addition, U.S. Pat. No. 4,582,985 discloses a fingerprintverification system made in an approximate size of a common credit card.

The semiconductor fingerprint sensor uses the difference of electricalcharacteristics according to the shape of valley and ridge of thefingerprint. When the fingerprint is contacted with the sensor, thefingerprint sensor senses electric signals induced by the electriccharacteristic of the fingerprint to extract the fingerprint pattern..

In case of the semiconductor fingerprint sensor, most components may beloaded on a semiconductor wafer, and easily made in a small size. Thesemiconductor fingerprint sensor in these days has a size as small as acoin, which is easily mounted to portable or small devices. In addition,since this sensor uses characteristics of the actual fingerprint, apicture or a mold of the fingerprint may not be possibly recognized,thereby ensuring the security system having higher security level thanthe optical one.

However, the semiconductor fingerprint sensor may have a weak durabilitysince it should be directly contacted with fingerprints.

FIG. 1 is a schematic functional diagram showing a general semiconductorfingerprint sensing apparatus.

Generally, the semiconductor fingerprint sensing apparatus 10 has afingerprint signal generating unit 20 for generating an analoguefingerprint signal (V_(SP)) expressing electric characteristics inherentto a fingerprint impedance according to the shape of valleys and ridgesof the fingerprint, a signal converter 30 for converting this analoguefingerprint signal (V_(SP)) into a digital signal, and a signalprocessing unit 40 for regularly processing the fingerprint signal inorder to detect or determine an inherent pattern of the fingerprint Anexample of such a semiconductor fingerprint sensing apparatus isdisclosed in U.S. Pat. No. 6,052,475 issued to Eric L. Upton in thetitle of “fingerprint detector using ridge resistance sensing array”.

In addition, this inventor has also developed a semiconductor sensingapparatus for generating an analogue fingerprint signal reflecting thecharging/discharging characteristic of the fingerprint impedance,counting and digitalizing the time required for this analogue signal toreach a reference level, and then detecting an inherent pattern of thefingerprint from this digital count value.

The fingerprint signal generating unit applied to the fingerprintsensing apparatus 10 developed by the inventor is configured as shown inFIG. 2.

This fingerprint signal generating unit 20 generates successive analoguefingerprint signals which reflect fingerprint impedances at each sensingpoint according to the fingerprint characteristics.

In other words, if a fingerprint to be detected is contacted on thesensing electrodes arranged in matrix, the fingerprint signal generatingunit 20 generates an analogue fingerprint signal (V_(SP)) expressing thecharging/discharging characteristics of the fingerprint impedance bysupplying or discharging charges to/from the fingerprint impedanceformed between the fingerprint and the sensing electrode.

This fingerprint signal generating unit 20 is also composed of a sensingelectrode 21, a fingerprint impedance 22, a parasitic impedance 23 and acharge supplying unit 24.

The sensing electrode 21 is directly contacted with the skin on whichthe fingerprint to be detected is formed. There are arranged a pluralityof sensing electrodes 21 on the surface of the fingerprint sensor inmatrix as shown in FIG. 3 a.

The fingerprint impedance 22 is formed between the sensing electrode 21and the valley or ridge of the fingerprint when the fingerprint iscontacted with the sensing electrode 21, as shown in FIG. 3 a. Thisfingerprint impedance 22 includes a resistance component (R_(F)) and acapacitance component (C_(F)), as shown in FIG. 3 b. In case of thefingerprint impedance, the difference of the resistances is generallygreat between the valley and the ridge of the fingerprint.

The parasitic impedance 23 is an inherent impedance of the sensingapparatus itself which is formed between the sensing electrode 21 and aground terminal (GND) when the fingerprint is not contacted with thesensing electrode 21. This parasitic impedance 23 includes a parasiticcapacitance significantly greater than the capacitance of thefingerprint impedance 22. Thus, the parasitic capacitance having agreater value than the capacitance of the fingerprint impedance becomesa main capacitance component of the sensing impedance (Z_(S);Z_(S)=Z_(F)+Z_(P)).

On the other hand, the parasitic resistance has a very small valuerather than the resistance of the fingerprint impedance 22, so theresistance of the fingerprint impedance 22 becomes a main resistancecomponent of the sensing impedance (Z_(S); Z_(S)=Z_(F)+Z_(P)).

The charge supplying unit 24 plays a role of charging or discharging theimpedance formed between the sensing electrode and the ground terminalby applying charges or cutting off the supply of charges to the sensingelectrode while the fingerprint is contacted with the sensing electrode.

In case of FIG. 2, a voltage source (V_(DD)) is adopted as the chargesupplying unit, and a digital switching element such as a tri-statebuffer 24 is used to control the operation of this voltage source(V_(DD)).

The voltage source (V_(DD)) may supply both a constant fixed voltage ora variable voltage.

The tri-state buffer 24 connects the voltage source to the sensingelectrode 21 and supplies charges to the fingerprint impedance 22 when“ON” signal is applied to the enable terminal (EN), while it quits thesupply of charges by cutting off the connection between the voltagesource and the sensing electrode 21 when “OFF” signal is applied to theenable terminal (EN).

As described above, in case the charges are directly applied to thefingerprint while the fingerprint is contacted with the sensingelectrodes 21, the finger and human body become directly exposed to DCcurrent. Thus, a large amount of current flows through the human body atonce from hundreds or thousands of sensing electrodes, which may beharmful to the human body. In addition, this causes great energyconsumption through the sensing electrodes during the initial chargingprocess.

DISCLOSURE OF INVENTION

The present invention is designed to solve the problems of the priorart, and therefore an object of the invention is to provide anfingerprint sensing apparatus which is capable of preventing DC currentfrom being directly applied between the finger (or, fingerprint) and asensing electrode of a fingerprint sensor.

In this reason, the fingerprint sensing apparatus includes a shieldingunit between a charge supplying unit and a sensing electrode forshielding the direct movement of DC current.

However, if the shielding unit is positioned between the chargesupplying unit and the sensing electrode, there may be arousedadditional problems.

In other words, the charges applied from the charge supplying unit dueto the voltages at both ends of the shielding unit may be nottransferred to the sensing electrode or the charges accumulated in thesensing electrode may be not discharged and remain, which causesresidual charges in the sensing electrode.

Thus, in order to solve the problem resulted from the adoption of theshielding unit, the present invention further uses an initiating unitfor setting identical initial conditions in the sensing electrodesand/or at both ends of the shielding unit.

In order to accomplish the above object, the fingerprint sensingapparatus of the present invention includes a plurality of sensingelectrodes arranged in matrix for contacting with a fingertip (or,ridges and valleys of the fingerprint); a charge supplying unit forsupplying charges to the sensing electrodes; and a shielding unitpositioned between the sensing electrode and the charge supplying unitfor preventing DC current from being directly transferred to thefingertip through the sensing electrode.

At this time, the shielding unit is preferably configured using a chargeaccumulating element.

Thus, it becomes possible to prevent damage of the human body caused bythe direct impression of DC current and reduce energy consumption of thesemiconductor fingerprint sensor by means of eliminating DC componentsof the electric signals generated when the fingerprint is contacted.

In another aspect of the present invention, there is also provided afingerprint sensing apparatus, which includes a plurality of sensingelectrodes arranged in matrix for contacting with a fingertip (or,ridges and valleys of the fingerprint); a charge supplying unit forsupplying charges to the sensing electrodes; a shielding unit positionedbetween the sensing electrode and the charge supplying unit forpreventing DC current from being directly transferred to the fingertipthrough the sensing electrode; and an initiating unit for keepingelectric characteristic of the sensing electrodes uniformly in order tosynchronize initial conditions for generating the fingerprint signals.

At this time, the initiating unit preferably includes a unit forintentionally eliminating residual charges existing in the sensingelectrodes and/or at both ends of the shielding unit after thedischarging of the sensing electrodes; and a unit for charging thesensing electrodes uniformly on the basis of the residual chargesexisting in the sensing electrodes after the discharging of the sensingelectrodes.

In still another aspect of the present invention, there is also provideda fingerprint sensing apparatus, which includes a plurality of sensingelectrodes arranged in matrix for contacting with a fingertip (or,ridges and valleys of the fingerprint); a charge supplying unit forsupplying charges to the sensing electrodes; a shielding unit positionedbetween the sensing electrode and the charge supplying unit forpreventing DC current from being directly transferred to the fingertipthrough the sensing electrode; and a discharging unit for intentionallydischarging the residual charges existing in the sensing electrodesand/or at both ends of the shielding unit in order to charge the sensingelectrodes uniformly.

At this time, the discharging unit may include a discharging path fordischarging the residual charges existing in the sensing electrodesand/or at both ends of the shielding unit to a ground point; and aswitching element installed on the discharging path to switch on/off.

In further another aspect of the present invention, there is alsoprovided a fingerprint sensing apparatus, which includes a plurality ofsensing electrodes arranged in matrix for contacting with a fingertip(or, ridges and valleys of the fingerprint); a charge supplying unit forsupplying charges to the sensing electrodes; a shielding unit positionedbetween the sensing electrode and the charge supplying unit forpreventing DC current from being directly transferred to the fingertipthrough the sensing electrode; a first discharging unit forintentionally discharging the residual charges existing at the sensingelectrodes in order to charge the sensing electrodes uniformly; and asecond discharging unit for intentionally discharging the residualcharges existing at both ends of the shielding unit in order to chargethe sensing electrodes uniformly.

At this time, the first discharging unit may include a first dischargingpath for discharging the residual charges existing in the sensingelectrodes to a ground point, and a first switching element installed onthe first discharging path to switch on/off; and the second dischargingunit may include a second discharging path for discharging the residualcharges existing at both ends of the shielding unit to a ground point,and a second switching element installed on the second discharging pathto switch on/off

Thus, the residual charges may be discharged to the ground point throughthe first and second discharging paths by switching on the first andsecond switching elements at the point that the residual charges aredischarged.

In still another aspect of the present invention, there is also provideda fingerprint sensing apparatus, which includes a plurality of sensingelectrodes arranged in matrix for contacting with a fingertip (or,ridges and valleys of the fingerprint); a charge supplying unit forsupplying charges to the sensing electrodes; a shielding unit positionedbetween the sensing electrode and the charge supplying unit forpreventing DC current from being directly transferred to the fingertipthrough the sensing electrode; a unit for measuring the residual chargesin the sensing electrodes; and a controller for controlling themagnitude of the current applied to the shielding unit from the voltagesource in order to compare the residual charge amount with a referencecharge amount and then compensate their difference.

In further another aspect of the present invention, there is alsoprovided a fingerprint sensing apparatus, which includes a plurality ofsensing electrodes arranged in matrix for contacting with a fingertip(or, ridges and valleys of the fingerprint); a charge supplying unit forsupplying charges to the sensing electrodes; a shielding unit positionedbetween the sensing electrode and the charge supplying unit forpreventing DC current from being directly transferred to the fingertipthrough the sensing electrode; a unit for measuring the residual chargesin the sensing electrodes; and a controller for controlling the timetaken for applying the current to the shielding unit from the voltagesource in order to compare the residual charge amount with a referencecharge amount and then compensate their difference.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of preferredembodiments of the present invention will be more fully described in thefollowing detailed description, taken accompanying drawings. In thedrawings:

FIG. 1 is a schematic functional diagram showing a general fingerprintsensing apparatus;

FIG. 2 shows detailed configuration of a fingerprint signal generatingunit applied to the conventional fingerprint sensing apparatus;

FIG. 3 a is a sectional view showing a skin resistance sensing array,and FIG. 3 b shows an equivalent circuit of a fingerprint impedance;

FIG. 4 shows configuration of a fingerprint signal generating deviceaccording to a first embodiment of the present invention;

FIG. 5 a is for illustrating the charging mechanism of the fingerprintsignal generating device of FIG. 4, FIG. 5 b is for illustrating thedischarging mechanism, and FIG. 5 c is for illustrating the electriccharacteristics of both ends of a shielding capacitor after discharging;

FIG. 6 shows configuration of the fingerprint signal generating deviceaccording to a second embodiment of the present invention;

FIG. 7 show configuration of the fingerprint signal generating deviceaccording to a third embodiment of the present invention; and

FIG. 8 is a timing chart showing the change of fingerprint signalaccording to the change of supplied charges and switching signal in thefingerprint signal generating circuits of FIGS. 6 and 7.

BEST MODES FOR CARRYING OUT THE INVENTION

Hereinafter, configuration of a fingerprint sensing apparatus accordingto preferred embodiments of the present invention will be described indetail with reference to the accompanying drawings.

The fingerprint signal generating device 100 and 200 described below isapplied to the fingerprint signal generating unit 20 of thesemiconductor fingerprint sensing apparatus 10 shown in FIG. 1 togenerate an analogue fingerprint signal reflecting the electriccharacteristics of the fingerprint impedance.

EMBODIMENT 1

At first, a fingerprint signal generating device according to the firstembodiment of the present invention is described with reference to FIG.4.

The fingerprint signal generating device 100 of this embodimentadditionally includes a shielding unit 120 between a charge supplyingunit (V_(PRE)) 110 and a sensing electrode 130. If the shielding unit120 is positioned between the tri-state buffer 110 and the sensingelectrode 130, the successive supply of DC current applied to the humanbody through the sensing electrode 130 may be cut off.

In case of FIG. 4, though the voltage source (V_(PRE)) and the tri-statebuffer 110 are exemplarily shown as the charge supplying unit, thecharge supplying unit of this embodiment is not limited to the case, andother charge supplying sources such as a current source may also beused, of course.

In addition, as for the shielding unit, this embodiment adopts acapacitor which is a kind of a charge accumulating element. However, theshielding unit of this embodiment is not absolutely limited to thecapacitor, other elements may also be used if they are capable ofaccomplishing the shielding function for preventing DC current frombeing directly applied to the human body.

Now, the operation of the fingerprint signal generating device of thisembodiment is described with reference to FIG. 4.

If the enable signal (EN) “ON” is applied to a gate terminal of thetri-state buffer 110, the fingerprint signal generating device 100 ofFIG. 4 may be expressed as the equivalent circuit of FIG. 5 a.

Due to the displacement current supplied from the voltage source(V_(PRE)), the (+) voltage of the shielding capacitor 120 increases upto a predetermined magnitude, and the (−) voltage of the capacitor 120thereby increases in concert with the (+) voltage.

The displacement current (I_(F)) having the cathode (−) is supplied tothe cathode (−) of the shielding capacitor 120 from the finger contactedwith the sensing electrode 130.

This displacement current (I_(F)) having the cathode (−) is successivelysupplied to the shielding capacitor 120 until the (−) voltage of theshielding capacitor is synchronized with the (+) voltage, in order toinitiate the voltage of the sensing electrode to a predetermined voltagelevel (or, to charge the sensing electrode).

On the other hand, if the enable signal “OFF” is applied to the gateterminal of the tri-state buffer, the fingerprint signal generatingdevice 100 of FIG. 4 may be expressed as the equivalent circuit of FIG.5 b.

In other words, the charges charged in the sensing electrode 130 aredischarged toward the ground point (GND2) through a fingerprintimpedance 140 due to the resistance of the fingerprint impedance 140, asshown in FIG. 5 b. This discharging characteristic of the sensingelectrode is output out of the fingerprint signal generating device 100as a fingerprint signal (V_(SP)). At this time, the dischargingcharacteristic of the fingerprint signal is mainly determined based onthe resistance of the fingerprint impedance.

As described above, since the DC component of the charges is notdirectly supplied to the human body from the voltage source or thecurrent source owing to the shielding capacitor 120, the fingerprintsignal generating device of this embodiment may be harmless to the humanbody and reduce the energy consumption dramatically.

However, though the shielding capacitor has an advantage that it mayprevent the DC current from being directly applied to the human body,there is also accompanied the following abnormal operations.

In other words, as shown in FIG. 5 c, if the charges of the sensingelectrode 130 are not all discharged, the residual charges may form acertain voltage (V_(a)) at both ends of the shielding capacitor 110.This phenomenon is always aroused whenever the time difference betweenframes are not sufficiently ensured. Thus, the residual charges areaccumulated in the sensing electrode at each frame, so the sensingelectrode resultantly has different offset voltages at each frame.Accordingly, the fingerprint signal (V_(SP)) cannot form a constantvoltage level, but there are formed fingerprint signals based on DCvoltage caused by the residual charges, which are different at eachframe. Thus, it becomes hardly possible to detect an accuratefingerprint signal.

If there exist residual charges in the sensing electrode or theshielding capacitor, it is impossible to uniformly initiate all sensingelectrodes of the fingerprint sensing apparatus to always have aconstant voltage level. Here, the term ‘uniform initiation’ is definedto identify electric states (or, initial voltage levels) of all sensingelectrodes before the charging in the cycle which periodically repeatscharging and discharging.

As described above, if the initiating state of the sensing electrodes isnot uniform, the pattern of the fingerprint signal changes according toa detect point or time of the signal, which makes it difficult to detecta reliable fingerprint pattern.

In order to solve this problem, the inventors adds an initiating unit tothe fingerprint signal generating device of the first embodiment. Thisinitiating unit is used for forcibly controlling initial voltage levelsof all sensing electrodes to have the same value before charging thesensing electrode for the generation of the fingerprint signal.

EMBODIMENT 2

At first, FIG. 6 shows a model for discharging the residual chargesexisting in the sensing electrode to the ground point throughdischarging paths formed in parallel to the shielding unit.

As shown in FIG. 6, the fingerprint signal generating circuit 200 ofthis embodiment includes a charge supplying unit 210, a shielding unit220, a sensing electrode 230, an initiating unit 260, a parasiticimpedance 250 and a fingerprint impedance 240.

The charge supplying unit is preferably a voltage source 210 forsupplying period pulse voltage as shown in FIG. 8. In addition, theshielding unit 220 is a capacitor which is a kind of a chargeaccumulating element, and the initiating unit 260 is configured with apath for electrically connecting both ends of the shielding capacitor220 and a switching element installed on the path. The switching elementis switched on/off according to a switching control signal (CSW) asshown in FIG. 6. This switching control signal (CSW) is applied from acontroller (not shown). Thus, the path becomes a short circuit when theswitching element becomes on, and the path functions as an open circuitwhen the switching element switches off.

The charge supplying unit and the shielding unit of the presentembodiment are not limited to the voltage source and the capacitor, andvarious equivalents may be adopted in order to accomplish the samefunction and the same effects.

In addition, the sensing electrode 230, the fingerprint impedance 240and the parasitic impedance 250 are configured identically to the firstembodiment.

Now, an initiating mechanism of the fingerprint signal generating deviceof this embodiment is described on the basis of the above-mentionedconfiguration with reference to FIGS. 6 and 8.

In the state that the fingerprint is contacted with the sensingelectrode 230, the switching element 260 becomes “OFF”, and thedischarging path is opened. The displacement current is applied from thevoltage source 210 to the anode (+) of the shielding capacitor 220, andthe (−) displacement voltage is applied to the cathode (−) of theshielding capacitor 220 from the fingertip of the human body so as to besynchronized with the increase of electric potential of the anode (+).Thereby, the sensing electrode 230 is initiated (or, charged) to apredetermined voltage level (V_(premax), see (a) and (c) of FIG. 8).

The charges charged in the sensing electrode 230 as above are mainlydischarged through the fingerprint impedance 240. An analoguefingerprint signal (V_(SP)) reflecting the discharging characteristic ofthe fingerprint impedance is output outside from the fingerprint signalgenerating device.

After a predetermined time from the initiation of the discharging, ifthe voltage level of the voltage source 210 is decreased to GND1, anelectric potential difference as much as V_(res) is generated betweenboth ends of the shielding capacitor 220. In addition, as the anode (+)of the shielding capacitor 220 is connected to GND1, the electricpotential difference of the sensing electrode (at the anode (−) of theshielding capacitor) is instantly reversed to −V_(res).

If the switching control pulse (see (b) of FIG. 8) is applied to theswitching element and the switching element switches on while thevoltage of the sensing electrode 230 is reversed, the path connects thesensing electrode to the ground point GND1 via the shielding capacitor.

If the discharging path is formed between the sensing electrode and theground point GND1 as described above, the residual charges existing inthe sensing electrode are discharged to GND1 through the dischargingpath.

If the switching element switches off and the path is again opened, allsensing electrodes keep uniform initial state until charges are suppliedagain from the voltage source (see (c) of FIG. 8).

EMBODIMENT 3

FIG. 7 shows a model for discharging the residual charges existing in,the sensing electrode to the ground point GND2 through a separatedischarging path.

As shown in FIG. 7, the fingerprint signal generating circuit 200 ofthis embodiment includes a charge supplying unit 210, a shielding unit220, a sensing electrode 230, an initiating unit 260, a parasiticimpedance 250 and a fingerprint impedance 240.

The charge supplying unit is preferably a voltage source 210 forsupplying period pulse voltage as shown in FIG. 8. In addition, theshielding unit 220 is a capacitor which is a kind of a chargeaccumulating element, and the initiating unit 260 is configured with apath for electrically connecting the sensing electrode to the groundpoint GND2 and a switching element installed on the path. The switchingelement is switched on/off according to a switching control signal (CSW)as shown in FIG. 6. This switching control signal (CSW) is applied froma controller (not shown). Thus, the path becomes a short circuit whenthe switching element becomes on, and the path functions as an opencircuit when the switching element switches off.

The charge supplying unit and the shielding unit of the presentembodiment are not limited to the voltage source and the capacitor, andvarious equivalents may be adopted in order to accomplish the samefunction and the same effects.

In addition, the sensing electrode 230, the fingerprint impedance 240and the parasitic impedance 250 are configured identically to the firstembodiment.

Now, an initiating mechanism of the fingerprint signal generating deviceof this embodiment is described on the basis of the above-mentionedconfiguration with reference to FIGS. 7 and 8.

In the state that the fingerprint is contacted with the sensingelectrode 230, the switching element 260 becomes “OFF”, and thedischarging path is opened. The displacement current is applied from thevoltage source 210 to the anode (+) of the shielding capacitor 220, andthe (−) displacement voltage is applied to the cathode (−) of theshielding capacitor 220 from the fingertip of the human body so as to besynchronized with the increase of electric potential of the anode (+).Thereby, the sensing electrode 230 is initiated (or, charged) to apredetermined voltage level (V_(premax), see (a) and (c) of FIG. 8).

The charges charged in the sensing electrode 230 as above are mainlydischarged through the fingerprint impedance 240. An analoguefingerprint signal (V_(SP)) reflecting the discharging characteristic ofthe fingerprint impedance is output outside from the fingerprint signalgenerating device.

After a predetermined time from the initiation of the discharging, ifthe voltage level of the voltage source 210 is decreased to GND 1, anelectric potential difference as much as V_(res) is generated betweenboth ends of the shielding capacitor 220. In addition, as the anode (+)of the shielding capacitor 220 is connected to GND1, the electricpotential difference of the sensing electrode (at the anode (−) of theshielding capacitor) is instantly reversed to −V_(res).

If the switching control pulse (see (b) of FIG. 8) is applied to theswitching element and the switching element switches on while thevoltage of the sensing electrode 230 is reversed, the path connects thesensing electrode to the ground point GND2.

If the discharging path is formed between the sensing electrode and theground point GND2 as described above, the residual charges existing inthe sensing electrode are discharged to GND2 through the dischargingpath.

If the switching element switches off and the path is again opened, allsensing electrodes keep uniform initial state until charges are suppliedagain from the voltage source (see (c) of FIG. 8).

The second and third embodiments arrange the shielding capacitor betweenthe voltage source and the sensing electrode in order to prevent DCcurrent from being directly supplied to the human body from the voltagesource. In addition, by discharging the residual charges remained in thesensing electrode or the shielding capacitor after discharging throughthe ground point GND1 or GND2, it is possible to keep the initialcharging level of all sensing electrodes to be always uniform.

The second and third embodiments make the initial condition of thesensing electrodes uniform by discharging the residual charges existingin the sensing electrodes to the ground point through a separatedischarging path..

However, the initiating unit for keeping the charging level of thesensing electrodes to be always uniform is not necessarily configuredusing the discharging path as in the embodiments. For example, it isalso possible to charge the sensing electrodes to always have a constantvoltage by, for example, measuring the charge amount remained in thesensing electrode or its voltage and then suitably controlling themagnitude of the current applied to the sensing electrode or the timetaken for applying the current.

As described above, the initiating unit of the present invention is notlimited to the case of the second or third embodiment, but various meansor methods may be adopted in order to accomplish the same function andresults.

INDUSTRIAL APPLICABILITY

The fingerprint sensing apparatus of the present invention may preventDC current from being directly applied to the human body by installing aseparate shielding unit between the sensing electrode and the current orvoltage source.

Thus, it becomes possible to fundamentally eliminate the factors givingharmful influences on the human body and reduce unnecessary energyconsumption in comparison to the prior one.

In addition, the fingerprint sensing apparatus of the present inventionis capable of preventing the charging level of the sensing electrodesfrom being changed and always initiating the electrodes uniformly byinstallation of the shielding unit.

The present invention has been described in detail. However, it shouldbe understood that the detailed description and specific examples, whileindicating preferred embodiments of the invention, are given by way ofillustration only, since various changes and modifications within thespirit and scope of the invention will become apparent to those skilledin the art from this detailed description.

1. A fingerprint sensing apparatus for generating successive analoguefingerprint signals reflecting a fingerprint impedance at each sensingpoint according to characteristics of a fingerprint in order to extractan inherent pattern of the fingerprint, the apparatus comprising: aplurality of sensing electrodes arranged in matrix for contacting with afingertip (or, ridges and valleys of the fingerprint); means forsupplying charges to the sensing electrodes; and shielding meanspositioned between the sensing electrode and the charge supplying meansfor preventing DC current from being directly transferred to thefingertip through the sensing electrode.
 2. A fingerprint sensingapparatus according to claim 1, wherein the shielding means is a chargeaccumulating element.
 3. A fingerprint sensing apparatus according toclaim 2, further comprising initiating means for keeping electriccharacteristic of the sensing electrodes uniformly in order tosynchronize initial conditions for generating the fingerprint signals.4. A fingerprint sensing apparatus according to claim 3, wherein theinitiating means is a discharging means for intentionally eliminatingresidual charges existing in the sensing electrodes and/or at both endsof the shielding means after the discharging of the sensing electrodes.5. A fingerprint sensing apparatus according to claim 4, wherein thedischarging means includes: a discharging path for discharging theresidual charges existing in the sensing electrodes and/or at both endsof the shielding means to a ground point; and a switching elementinstalled on the discharging path to switch on/off.
 6. A fingerprintsensing apparatus according to claim 3, wherein the initiating means isa means for charging the sensing electrodes uniformly on the basis ofthe residual charges existing in the sensing electrodes after thedischarging of the sensing electrodes.
 7. A fingerprint sensingapparatus for generating successive analogue fingerprint signalsreflecting a fingerprint impedance at each sensing point according tocharacteristics of a fingerprint in order to extract an inherent patternof the fingerprint, the apparatus comprising: a plurality of sensingelectrodes arranged in matrix for contacting with a fingertip (or,ridges and valleys of the fingerprint); means for supplying charges tothe sensing electrodes; shielding means positioned between the sensingelectrode and the charge supplying means for preventing DC current frombeing directly transferred to the fingertip through the sensingelectrode; and initiating means for keeping electric characteristic ofthe sensing electrodes uniformly in order to synchronize initialconditions for generating the fingerprint signals.
 8. A fingerprintsensing apparatus according to claim 7, wherein the initiating means isa discharging means for intentionally eliminating residual chargesexisting in the sensing electrodes and/or at both ends of the shieldingmeans after the discharging of the sensing electrodes.
 9. A fingerprintsensing apparatus according to claim 8, wherein the discharging meansincludes: a discharging path for discharging the residual chargesexisting in the sensing electrodes and/or at both ends of the shieldingmeans to a ground point; and a switching element installed on thedischarging path to switch on/off.
 10. A fingerprint sensing apparatusaccording to claim 7, wherein the initiating means is a means forcharging the sensing electrodes uniformly on the basis of the residualcharges existing in the sensing electrodes after the discharging of thesensing electrodes.